The Golgi apparatus plays a key role in protein sorting and glycoslyation within the eukaryotic secretory pathway. Defects in vesicular trafficking within the Golgi affect both its structure and function. As a consequence, such defects can have pleiotropic effects on the glycosylation and stability of cell surface proteins, leading to human disease. The current proposal focuses on a recently discovered protein that is essential for normal Golgi morphology and function. This hetero-octameric protein, known as the Conserved Oligomeric Golgi (or COG) complex, appears to be important for retrograde trafficking within the Golgi apparatus, where it likely functions to direct vesicles originating from the trans-Golgi to cis Golgi cisternae. Furthermore, COG is a member of a family of intracellular trafficking proteins known as tethering factors. Several lines of evidence indicate that tethering factors act upstream of SNAREs, mediating the earliest contact between transport vesicles and their membrane targets. Nonetheless tethering factors, despite their central importance, are relatively poorly understood. Specifically, almost nothing is known about the structure of any of the six large hetero-oligomeric tethering proteins, including COG, thought to function in the secretory pathway, nor has their mechanism of action been well characterized. This proposal, therefore, aims to begin filling in this gap by undertaking coarse- and fine-grained structural studies of COG. In the first aim, a complementary set of biochemical methods will be used to elucidate the subunit connectivity and overall architecture of the complex. In the second aim, proteins that interact physically and functionally with COG will be identified using immunoprecipitation with a battery of polyclonal antibodies. Aims 3 and 4 initiate high-resolution structural analysis of COG. In the third aim, the structure of the yeast Cog2p subunit will be elucidated using two- and three-dimensional NMR. In the final aim, X-ray crystallography will be used to determine the structures of larger COG subassemblies and complexes with other trafficking factors. [unreadable] [unreadable]